Combustors are commonly used to ignite fuel to produce combustion gases having a high temperature and pressure. Combustor nozzles typically include a body or a downstream surface at a nozzle tip, and a working fluid and/or fuel is supplied through the nozzle tip to a combustion chamber where the combustion occurs. The temperature difference between the working fluid and fuel on one side of the nozzle tip and the combustion gases on the other side of the nozzle tip creates a substantial thermal gradient across the nozzle tip that may produce cracking or premature failure in the nozzle tip. As a result, the nozzle tip is often forged from metal alloys and may also be coated with a thermal barrier coating to enhance fatigue resistance to cracking Alternately or in addition, cooling holes or passages may be formed through the nozzle tip to allow a portion of the working fluid and/or fuel to pass through the nozzle tip to cool the downstream surface and reduce the temperature difference across the nozzle tip.
The holes or passages may be machined into the nozzle tip using various methods known in the art. For example, electron discharge machining (EDM) may be used to melt the forged metal alloy to create the holes or passages. However, the high temperatures associated with the EDM process leaves a recast layer inside the holes or passages, and the recast layer is typically substantially less resistant to fatigue cracking than the original forged metal alloy. In addition, holes and passages that are angled with respect to an axial centerline of the nozzle tip to enhance cooling to the nozzle tip may result in unsupported portions of the nozzle tip that are more susceptible to fatigue cracking. Although in many cases, the additional cracking caused by the recast layer and/or unsupported portions is merely cosmetic, severe cracking may lead to material loss from the nozzle tip and possible downstream damage. Therefore, an improved combustor nozzle and/or method for modifying the combustor nozzle that enhances resistance to fatigue cracking would be useful.